You lose 1% chance of not being able to do double damage during Samba.
Put them on and throw/shoot something in a starter zone. You can calculate the damage you should get and compare it to the damage you got to determine your critical hit damage bonus.
What would be the default hit crit pDIF should I be looking at? 2.523? (According to formula written in BGWiki)
EDIT : /facepalm, forgot I have RNG. Things can be easily tested that way.
It's 4% crit dmg boost, definitely not 3% or 5%
429 normal hit with harrier +2 + wooden arrow,
777 crit without pants. floor((floor(429*1.25)*1.45 dead aim) = 777
798 crit with pants.floor((floor(429*1.25)*1.49 dead aim+pants) = 798